The free surface of the serous membranes separates entirely from the neighbouring organs those upon which these membranes are spread; so that they are to these organs real boundaries, barriers, if I may use the term, or integuments, if it should be preferred, very different however from those which are external. Observe in fact that all the principal viscera, the heart, the lungs, the brain, the gastric viscera, the testicles, &c. limited by their serous covering, suspended in the middle of the sac that it forms, only communicate with the adjacent parts where their vessels enter; everywhere else there is contiguity and not continuity. This insulation of position coincides very well with the insulation of vitality which is remarked in all the organs, and especially in those that we have just noticed. Each has its peculiar life, which is the result of a particular modification of its vital forces, a modification which necessarily establishes one in the circulation, nutrition and temperature. No part feels, is moved and nourished like another, unless it belongs to the same system. Each organ executes on a small scale the phenomena which take place on a large one in the economy; each takes from the circulation the aliment that is proper for it, digests it, throws back into the mass of blood, the portion which is heterogeneous to it, and appropriates to itself that which can nourish it; it is digestion in miniature. No doubt the ancients wished to give an idea of this truth which has been so well explained by Bordeu, when they said that the womb was a living animal within another. A very important use then of the serous membranes is to contribute, by rendering independent the position of their respective organs, to the independence of the vital forces, life and functions of these organs. Let us not forget to consider under the same point of view, the moist atmosphere with which they are constantly surrounded, an atmosphere analogous to that which the cellular texture forms for various other organs. In this atmosphere all the morbific emanations of the organ go and are lost, if we may so say, without these emanations injuring the other organs. We have seen that this atmosphere in the cellular system is sometimes the seat of phenomena wholly different, and serves to transmit diseases from one organ to another. Now the serous membranes are a barrier much less easily surmounted, because they have not filaments which go from one organ to another, there is only contiguity as I have said, with the organs that they surround. We very rarely see in the abdomen a disease of the liver communicated to the intestines, one of the spleen passing to the stomach, &c.